The present invention relates to a portable stencil making device, and more particularly, to such device employing a stencil sheet perforatable by irradiation of infrared rays or by a thermal head and having means for controlling dot perforation density with respect to the stencil sheet.
Conventionally, has been known a small sized portable stencil making device employing a stencil sheet perforated by a stencil pen or a ball point pen to form a perforated image. One such conventional stencil making device is described in a Japanese Patent Application (Kokoku) No. 54-9523 as shown in FIG. 1. The stencil making device includes a table 100 and an ink applicator unit 102. The ink applicator unit 102 reciprocably supports the stencil sheet 104 to perform mimeographic printing at one extreme or protruded position, and is rested in another extreme or retracted position.
Further, has been known a conventional heat sensitive stencil sheet perforatable by infrared rays or a thermal head. One typical example of such sheet includes a thermoplastic resin film and a porous thin substrate adhered thereto by an adhesive agent. One conventional example of a stencil making device employing such heat sensitive stencil sheet is described in a Japanese Patent Application Kokai (OPI) No. 63-17074 as shown in FIG. 2. According to the conventional device, a stencil sheet 106 has a perforated heat sensitive stencil paper 110 and an ink impermeable cover sheet 112 superposed with each other and bonded to a frame 108. An ink is retained between the heat sensitive stencil paper 110 and the cover sheet 112. Further, The stencil sheet 106 retaining therein the ink is affixed to a stand 114 provided with a grip member 120 and having a cushion layer 116 and an adhesive layer 118. By pressing the grip member 120 against a print sheet, mimeographic printing is achieved through the stencil sheet 110.
However, as shown in FIG. 3, a perforation 51 will provide an ink enlarging area 52 spreading from the perforation 51. In case of the former type of the stencil making device, if large pressure is applied, a large volume of ink may pass through the heat sensitive stencil sheet 110 at an area where perforations are highly densely formed. Therefore, large volume of ink reaches the print sheet. As a result, it takes a long period for drying the thus transferred ink corresponding to that area, and the ink may further spread to blur an inked image.
If a shadow character H is to be printed as shown in FIG. 4, portions C and D are shown as enlarged views in FIGS. 5(a) and 5(b) in which 4.times.4 ON dots patterns are shown for comparison. As is apparent from these FIGURES, the portion D provides larger ink superposing portions than the portion C. That is, in the portion D, there are 4 (four) single ink portion, 12 (twelve) double piled ink portions, and 9 (nine) four-piled ink portions. In other words, there are many ink overlapping portions.
With such an arrangement, it takes a long time for drying the deposited ink, and therefore, long period is required for finishing a print, and moreover, ink blurring may occur. Further, operator's hand may be contaminated with the excessive ink prior to the ink drying, and non-printed area may also be contaminated by the undried ink.